I. Field of the Invention
This invention relates generally to the field of data communication systems, and more particularly to a logic circuit board used therein which can be inserted into and extracted from a backplane without removing power from the system, or interfering with data transfer in the system.
II. Description of the Prior Art
Data and network communication systems typically require a long up-time when initialized. This is particularly true when a large number of logic boards are connected and integrated into the system. Communication systems, in particular, also require a long period of time for initialization. These systems typically include a plurality of logic boards inserted into boards slots of a card cage, each card interfacing with an edge connector on a backplane or motherboard. Conventionally, to insert or remove a circuit board from the system backplane, the chassis would ordinarily be first powered down before insertion or removal of a circuit board. This has been necessary to avoid the sudden making or breaking of circuit connections, which tends to generate electrical noise on the backplane power bus and causing errors on the data bus. The reconfiguration and initialization of the system has necessitated long and inconvenient system down-time.
Recently, live-insertion of circuit boards into a card chassis, commonly referred to as "hot-swapping" the boards, has been explored. Standardized methodology through which a faulty board can be swapped out of a system, and a replacement board swapped in, while the system remains up and running is only now being established. Several vendors now offer live-insertion of circuit modules without damaging the module itself, or interfering with data communications of the system.
One such device is a hot extraction and insertion of logic boards in an on-line communication system, disclosed in U.S. Pat. No. 4,999,787 to McNally et al. and assigned to Bull H. N. Information Systems, Inc. A defective logic board can be replaced, or installed into the system, without powering down the system, by providing external auxiliary voltages and a reset signal to the board to put it in a passive state while it is being plugged into or unplugged from its backplane connector. This approach is the so-called umbilical cord approach. However, the idea of providing power to a circuit board removed from a chassis creates the possibility the circuit board will become damaged if it accidentally comes into contact with a metal surface resulting in a short. Further, the bus activity needs to be halted as a circuit board is inserted or removed from the backplane connector, thus making the system temporarily inoperable. Hence, this live-insertion technique is not transparent to the system.
U.S. Pat. No. 4,835,737 to Herrig, et al. and assigned to AT & T, teaches a method and apparatus for controlled removal and insertion of circuit modules. A latch activated switch provides a control signal in anticipation of circuit board removal. The control signal activates a finite state machine, which seizes control of the bus after completion of any current bus communications, and stops the generation of clock pulses normally required in bus communications. When contact is physically broken between the board and its corresponding connector, the finite state machine restores the bus clock pulses and relinquishes control of the bus. Similarly, when a board is to be inserted into a slot and connected to an open connector, contact between the board and the connector is sensed by the finite state machine which causes the bus to be seized and the bus clock pulses to be temporarily inhibited. When the board is fully inserted, the finite state machine restores the bus clock pulses and relinquishes control of the bus. This device is similar to the device taught by McNally et al., in that the system needs to be halted during the insertion or extraction process, thus rendering the system temporarily inoperative.
U.S. Pat. No. 4,750,136 to Arpin et al. and also assigned to AT & T, teaches a communication system having automatic circuit board initialization capability. A malfunctioning circuit board can be replaced while the system is operating, and the replacement circuit board is automatically initialized by the system using the stored operating parameters from the malfunctioning circuit board. However, these teachings are only generally related to auto-configuration.
U.S. Pat. No. 3,993,935 to Phillips et al. and assigned to the Xerox Corporation, teaches a printed circuit board connection in which some contacts on the board edge are recessed slightly from the edge so that certain electrical connections may be made prior to the others. However, when the circuit board power busses eventually make contact with the backplane power bus, a surge of power is generated on the circuit board, which causes noise on both the backplane power bus and the circuit board power bus. This noise is a primary source of data errors of the system. There is no graceful application or removal of power to and from the circuit board module, and this board is not suitable for live insertion in a card-cage chassis.
While each of these devices provide a live-insertion or "hot-swapping" capability of the circuit card modules, none allows for hot-swapping circuit cards in a way that is transparent to the system, i.e. does not temporarily halt data bus communications, or does not require modification of the backplane, and which allows for insertion and extraction of circuit board modules without first performing initial steps.